Electric vehicle power distribution system

ABSTRACT

A power distribution system comprises a plurality of energy storage modules coupled in series. A plurality of electrically isolated power converters are each coupled across one or more of the energy storage modules. When enabled, the power converters provide a low voltage output to an output of the power distribution system. A control system controls the power converters to provide the low voltage output. The control system selectively enables the power converters to balance states of charge of the energy storage modules.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/117,822, filed Feb. 18, 2015, which is incorporated herein byreference in its entirety.

BACKGROUND

This disclosure relates to power distribution systems for electricvehicles.

Battery-powered vehicles commonly use batteries to supply both a highoutput voltage for powering the vehicle and low output voltages foroperating various computers, lights, electric fans, and other low-powersystems on board the vehicle. To provide the low voltage to the on-boardsystems, the batteries often include a voltage converter to convert thehigh output voltage to a lower voltage. The low-voltage power output ofvehicle batteries is often a small fraction of the nominal power outputof the battery. Because the amount of time the batteries operate inlow-voltage modes can be very long, it is desirable for voltageconverters to operate efficiently to reduce loss of energy stored in thebatteries. However, the efficiency of voltage converters decreases asthe difference between the input voltage and the output voltageincreases.

To improve efficiency in low voltage modes, power systems forbattery-powered vehicles may include an auxiliary battery having asuitable voltage output for operating the on-board systems without aconverter. If the efficiency of a voltage converter for the high-voltagepropulsion batteries drops below a threshold value, the auxiliarybattery is used to supply power to the on-board systems. The auxiliarybattery may be charged by the voltage converter of the high-propulsionbatteries. However, an auxiliary battery represents a single point offailure for the power systems of the electric vehicle. A secondauxiliary battery and voltage converter may be added to the power systemto provide redundancy, but this redundancy adds weight to the vehicle.

SUMMARY

A power distribution system of an electric vehicle provides a highoutput voltage to propulsion systems of the electric vehicle and a lowoutput voltage to on-board systems of the electric vehicle. In oneembodiment, the power distribution system comprises a plurality ofenergy storage modules coupled in series and a plurality ofelectrically-isolated power converters each coupled across one or moreof the energy storage modules. When enabled, the power convertersprovide a low voltage output to an output of the power distributionsystem, such as the on-board systems of the electric vehicle.

A control system controls the power converters of the power distributionsystem to provide the low output voltage. The control system selectivelyenables the power converters to balance power output of the energystorage modules. In one embodiment, the control system selectivelyenables power converters based on a state of charge of each of theenergy storage modules. The control system may obtain a directmeasurement of the state of charge of each of the modules, or mayestimate the states of charge based on charge withdrawn from each of themodules or an amount of time the power converters corresponding to themodules have been enabled.

The features and advantages described in the specification are not allinclusive and, in particular, many additional features and advantageswill be apparent to one of ordinary skill in the art in view of thedrawings, specification, and claims. Moreover, it should be noted thatthe language used in the specification has been principally selected forreadability and instructional purposes, and may not have been selectedto delineate or circumscribe the inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a power distribution system 100, accordingto one embodiment.

FIG. 2 illustrates an example DC power converter, according to oneembodiment.

FIG. 3 is a plot illustrating an example desired operating region of aDC power converter.

FIG. 4 is a flowchart illustrating a method for controlling DC powerconverters, according to one embodiment.

The figures depict various embodiments of the present invention forpurposes of illustration only. One skilled in the art will readilyrecognize from the following discussion that alternative embodiments ofthe structures and methods illustrated herein may be employed withoutdeparting from the principles of the invention described herein.

DETAILED DESCRIPTION

Overview

The figures and the following description relate to various embodimentsby way of illustration only. It should be noted that from the followingdiscussion, alternative embodiments of the structures and methodsdisclosed herein will be readily recognized as viable alternatives thatmay be employed without departing from the principles of the claimedinvention.

Reference will now be made in detail to several embodiments, examples ofwhich are illustrated in the accompanying figures. It is noted thatwherever practicable similar or like reference numbers may be used inthe figures and may indicate similar or like functionality. The figuresdepict embodiments of the present invention for purposes of illustrationonly. One skilled in the art will readily recognize from the followingdescription that alternative embodiments of the structures and methodsillustrated herein may be employed without departing from the principlesof the invention described herein.

FIG. 1 is a block diagram of a power distribution system 100, accordingto one embodiment. The power distribution system 100 is suitable for usein a battery-powered vehicle, such as an electric car or aircraft. Inone embodiment, the power distribution system 100 includes a pluralityof energy storage modules 110A-C (collectively, energy storage modules110), battery management systems 120A-C, and a plurality of powerconverters 130. Other embodiments of the power distribution system 100may include additional or different components.

The energy storage modules 110 are coupled in series and collectivelysupply a high voltage output V_(High) to propulsion systems of thebattery-powered vehicle. Each energy storage module 110 includes one ormore battery cells. For example, one embodiment of an energy storagemodule 110 has 12 battery cells each supplying a voltage ofapproximately 3V to 4.2V, resulting in a voltage range for the energystorage module 110 of approximately 36V to 50V. Although FIG. 1illustrates only three energy storage modules 110 of the powerdistribution system 100, the power distribution system 100 may have anynumber of energy storage modules 110 for supplying V_(High) to thepropulsion systems. For example, one embodiment of the powerdistribution system 100 includes twelve energy storage modules 110.

In one embodiment, the energy storage modules 110 are managed by aplurality of battery management systems 120. The battery managementsystems 120 monitor a state of the modules 110. The state of the modules110 may include the voltage of all the modules' battery cells, thecurrent output of the modules 110, temperature of the modules 110, thestate of charge of the module 110, and other factors influencing theoverall health of the modules 110.

The power converters 130 are coupled across one or more of the energystorage modules 110 and convert DC voltage output of the energy storagemodules 110 to a desired DC low output voltage compatible with on-boardsystems of the battery-powered vehicle. When enabled, each powerconverter 130 is configured to produce an output voltage compatible withvoltage specifications of the on-board systems by stepping down thevoltage of the energy storage module or modules 110 corresponding to therespective power converter 130 to the desired low output voltage. Whendisabled, a power converter 130 does not provide current to the on-boardsystems. In one embodiment, the power converters 130 are configured tocollectively supply a peak low power output to the on-board systems(e.g., when a maximum number of the on-board systems are turned on andeach is supplying its maximum current), while a subset of the powerconverters 130 are enabled during operating modes requiring less power.In another embodiment, the each power converter 130 is configured tostep up the voltage of the energy storage modules 110 to a low voltageoutput V_(Low). In still another embodiment, the each power converter130 is configured to step up or down the voltage of the energy storagemodules 110 to a low voltage output V_(Low), according to the state ofcharge and load on the energy storage modules 110.

The power converters 130 may be incorporated into the battery managementsystems 120, as shown in FIG. 1, or may be external to the batterymanagement systems 120. In one embodiment, the power converters 130 orthe battery management system 120 monitor health of the power converters130 and the energy storage modules 110. For example, the batterymanagement system 120 monitors internal resistance, energy storagecapacity, voltage, self-discharge, and number of charge/discharge cyclesof the energy storage modules 110 to analyze the state of healthy of themodules 110, and monitors under- and over-voltage conditions of thepower converters 130. If the battery management system 120 finds anyconverters 130 or energy storage modules 110 to be faulty, the faultyconverters or the converters corresponding to the faulty modules 110 aredeactivated. Alternatively, each of the power converters 130 comprises afuse configured to disconnect the power converter 130 from the lowvoltage output of the power distribution system 100 during a faultcondition. Furthermore, as the voltage across the energy storage modules110 may be different and the power converters 130 are coupled to acommon output, the power converters 130 are galvanically isolated.

The controller 140 selectively enables the power converters 130 toprovide a low voltage output V_(Low). In one embodiment, the controller140 selects one or more modules 110 to provide the low voltage outputV_(Low), to efficiently operate the power distribution system 100.Moreover, the controller 140 may select the modules 110 for providingoutput power such that the state of charge of the modules 110 of powerdistribution system 100 is kept balanced within a desired range. Thecontroller 140 enables the power converters 130 coupled across theselected modules, which convert the voltage output of the selectedmodules to V_(Low). The controller 140 may select the modules to beenabled in a variety of different manners. In one embodiment, thecontroller 140 selectively enables power converters 130 based on statesof charge of the modules 110 corresponding to each of the powerconverters 130. In another embodiment, the controller 140 selectivelyenables power converters 130 such that product of the amount of outputcurrent and the duration of it being enabled is substantially equalacross all modules 110. In this way, the amount of energy withdrawn fromeach module will be substantially equal, and the states of charge of themodules will remain balanced as they discharge.

FIG. 2 illustrates an example power converter 130. The example converter130 shown in FIG. 2 is configured in a half-bridge topology, althoughthe power converter 130 may alternatively be configured according toother converter topologies having galvanically isolated outputs (e.g.,flyback or buck topologies). In the embodiment illustrated in FIG. 2,the power converter 130 comprises switches 202A and 202B and atransformer 204. The switches 202 turn on or off alternately to generatea current in a primary coil of the transformer 204. The voltage acrossthe primary coil of the transformer 204 generates a correspondingcurrent in a secondary coil of the transformer 204, creating a voltageacross the capacitor C1 that is output to the low voltage output. In oneembodiment, switching of the switches 202 is controlled by thecontroller 140 to generate a desired output voltage from the powerconverter 130. The transformer 204 galvanically isolates the output ofthe power converter 130 from the energy storage module 110, and one ormore diodes on the secondary side of the transformer 204 isolate theplurality of power converters 130 of the power distribution system 100.

The controller 140 receives an output current of the converter 130 froman ammeter 216, output voltage, and a voltage across the module 110corresponding to the converter 130. The controller 140 controlsswitching of the switches 202 based on the output current and thevoltage of the module 110 to regulate the output voltage V_(Low). In oneembodiment, as shown in FIG. 2, the controller 140 comprises a modulebalancer 210 and one or more current control blocks 212.

The module balancer 210 receives a measure of the voltage input to thepower converter 130 from a voltmeter 214 of the battery managementsystem 120. For example, if the power converter 130 is coupled acrossone module 110 of the power distribution system 100 as shown in FIG. 2,the module balancer 210 receives the module's voltage from the voltmeter214 coupled across the module 110. Although FIG. 2 illustrates themodule balancer 210 receiving the voltage from a single voltmeter 214,the module balance 210 may receive the voltage measured by voltmeterscoupled across each of the modules 110 or sets of the modules 110. Themodule balancer 210 also receives the low output voltage V_(Low) asfeedback. The module balancer 210 regulates the low output voltage orthe output power based on the feedback. In one embodiment, the modulebalancer 210 determines a number of energy storage modules 110 to enableto achieve the desired low output voltage V_(Low), and generates acontrol signal input to the one or more current control blocks 212 toenable the selected number of modules 110.

As the energy storage modules 110 may have different states of charge ata given time due to variability in internal resistance and differentamounts of energy storage capacity, the module balancer 210 selectivelyenables the power converters 130 to balance the modules 110 during thecourse of operation of the power distribution system 100. The modulebalancer 210 balances the modules 110 by selectively activating powerconverters 130 based on states of charge of the modules 110. In oneembodiment, the module balancer 210 receives the states of charge of themodules 110 from the battery management systems 120, and enables one ormore power converters 130 corresponding to modules 110 having higherstates of charge. However, as the battery management systems 120 may notbe configured to directly measure the states of charge of the modules110, one embodiment of the module balancer 210 estimates the states ofcharge based on current output from the power converters 130. Forexample, the module balancer 210 determines an amount of chargewithdrawn from the modules 110 corresponding to each power converter 130based on the current output of the power converter 130 over time. Inanother case, the module balancer 210 enables the power converters 130for equal periods of time. For example, the module balancer 210 monitorsan amount of time each of the power converters 130 is enabled androtates through the power converters 130 to enable the power converters130 for approximately the same length of time. One embodiment of aprocess for balancing the modules 110 is described with respect to FIG.4.

The current control circuit 212 receives the control signal generated bythe module balancer 210 to selectively activate one or more of the powerconverters 130. Based on the control signal, the current control circuit212 generates control signals to turn on and turn off the switches 202of the power converter 130. For example, if the module balancer 210generates a control signal to enable a particular power converter 130,the current control circuit 212 generates signals to turn on and off theswitches 202 in response to receiving the control signal. In oneembodiment, the controller 140 includes a current control circuit 212for each power converter 130 of the power distribution system 100. Thecurrent control circuit 212 receives a signal from the ammeter 216 thatis indicative of the current output by the power converter 130. Thecurrent control circuit 212 drives the switches 202 to regulate currentoutput from the power converter 130.

In one embodiment, the current control circuit 212 regulates the outputcurrent from each of the power converters 130 to operate each powerconverter 130 within a desired operating region. FIG. 3 is a plotillustrating an example desired operating region 302. As shown in FIG.3, efficiency of the power converter 130 is a function of the outputcurrent. The current control circuit 212 regulates the current output tooperate between a lower current threshold 304 and an upper currentthreshold 306. In one embodiment, the lower current threshold 304 is thecurrent output at which the efficiency of the converter 130 falls belowa threshold efficiency 308. The upper current threshold 306 may be anupper rating for current output of the power converter 130 or a currentoutput at which the efficiency of the converter 130 falls below athreshold efficiency. The upper and lower current thresholds mayalternatively be defined in numerous other ways.

As described above, the controller 140 selectively enables the powerconverters 130 to output a low voltage V_(Low), from one or more of theenergy storage modules 110. One embodiment of a method used by thecontroller 140 to control the power converters 130 is shown in FIG. 4.In the embodiment illustrated in FIG. 4, the controller 140 selectivelyenables the power converters 130 based on an amount of charge withdrawnfrom the modules 110 as an estimate of the state of charge of each ofthe modules 110. In other embodiments, the controller 140 uses otherproperties of the modules 110 to selectively enable the converters 130.For example, one embodiment of the controller 140 receives a state ofcharge of each module 110 from the battery management system 120corresponding to the module 110 and determines which power converters130 to enable and which to disable to provide a desired low outputvoltage V_(Low).

Referring to FIG. 4, the controller 140 determines 401, based on thebattery charging and balancing process, an initial state of charge. Thecontroller 140 then determines 402 the amount of energy withdrawn fromeach of the modules 110. In one embodiment, the controller 140 monitorsthe current output from each module 110 as the controller 140 drives themodules 110 to supply a low output voltage, and determines the state ofcharge of the module by subtracting the amount of energy withdrawn fromthe modules 110 from the state of charge estimated during the last timethe modules 110 were charged.

Based on the amount of current withdrawn from each of the modules 110,the controller 140 enables 404 one or more power converters 130corresponding to the modules 110 having lower amounts of chargewithdrawn. For example, the controller 140 sorts an array correspondingto the modules 110 according to the amount of charge withdrawn from eachof the modules, and selects one or more of the modules 110 in the arrayhaving the lowest charge output. Alternatively, if the controller 140has a state of charge estimate of the modules 110, the controller 140enables one or more power converters 130 corresponding to the modules110 having the highest estimated states of charge.

The controller 140 measures 406 the current output of the enabledconverters and compares the output current to upper and lower thresholdvalues. If the current output is greater 408 than the upper threshold,the controller 140 increases 410 the number of enabled converters todecrease the current contributed by each enabled module 110. Incontrast, if the current output is less than the upper threshold andless than the lower threshold 412, the controller 140 decreases 414 thenumber of enabled converters to increase the current contributed by eachenabled module 110. If the current output is between the upper and lowerthresholds, the controller 140 continues monitoring the current outputand increases or decreases the number of enabled power converters 130 ifthe output current is not between the thresholds. Accordingly, thecontroller 140 maintains the output current from the enabled powerconverters 130 between upper and lower thresholds, such as the upper andlower thresholds 306 and 304 defining the desired operating range 302shown in FIG. 3.

While particular embodiments and applications have been illustrated anddescribed herein, it is to be understood that the embodiments are notlimited to the precise construction and components disclosed herein andthat various modifications, changes, and variations may be made in thearrangement, operation, and details of the methods and apparatuses ofthe embodiments without departing from the spirit and scope of theembodiments as defined in the appended claims.

Upon reading this disclosure, those of skill in the art will appreciatestill additional alternative designs for the system. Thus, whileparticular embodiments and applications of the present invention havebeen illustrated and described, it is to be understood that theinvention is not limited to the precise construction and componentsdisclosed herein and that various modifications, changes and variationswhich will be apparent to those skilled in the art may be made in thearrangement, operation and details of the method and apparatus of thepresent invention disclosed herein without departing from the spirit andscope of the invention as defined in any claims drawn to the subjectmatter herein.

What is claimed is:
 1. A power distribution system, comprising: aplurality of energy storage modules coupled in series; a plurality ofelectrically isolated power converters, each coupled across one or moreof the energy storage modules, the power converters when enabledconfigured to provide a low voltage output to an output of the powerdistribution system; and a control system controlling the powerconverters to provide the low voltage output, the control systemconfigured to selectively enable one or more of the power converters tobalance states of charge of the energy storage modules, wherein thecontrol system is configured to balance states of charge of the energystorage modules including by: monitoring a current output from a firstselectively enabled power converter, in the one or more selectivelyenabled power converters, in order to obtain a first power convertercurrent output; monitoring a current output from a second selectivelyenabled power converter, in the one or more selectively enabled powerconverters, in order to obtain a second power converter current output;enabling the first selectively enabled power converter for a firstduration of time; and enabling the second selectively enabled powerconverter for a second duration of time, wherein a first product of thefirst power converter current output and the first duration of timesubstantially equals a second product of the second power convertercurrent output and the second duration of time.
 2. The powerdistribution system of claim 1, wherein the control system is configuredto selectively enable the one or more power converters based on thecurrent output by: determining an amount of charge withdrawn from theenergy storage modules based on the current output from the powerconverters; and enabling one or more power converters corresponding toenergy storage modules having lower amounts of charge withdrawn.
 3. Thepower distribution system of claim 1, wherein the control system isconfigured to selectively enable power converters to maintain thecurrent output within a desired operating range of the power converter.4. The power distribution system of claim 3, wherein the control systemis configured to maintain the current output within the desiredoperating range of the power converter by: increasing a number ofenabled power converters responsive to the current output being above anupper threshold of the desired operating range; and decreasing thenumber of enabled power converters responsive to the current outputbeing below a lower threshold of the desired operating range.
 5. Thepower distribution system of claim 1, wherein the control system isconfigured to balance states of charge of the energy storage modules by:determining a state of charge of the energy storage modules; andselectively enabling one or more power converters corresponding toenergy storage modules having higher states of charge.
 6. The powerdistribution system of claim 1, wherein the control system is configuredto balance states of charge of the energy storage modules by: monitoringan amount of time each power converter is enabled; and selectivelyenabling the power converters for substantially equivalent amounts oftime.
 7. The power distribution system of claim 1, wherein each of thepower converters comprises: a transformer including a primary coil and asecondary coil, the primary coil coupled to the one or more energystorage modules corresponding to the power converter and the secondarycoil coupled to the output of the power distribution system; and one ormore diode coupled between the secondary coil and the output of thepower distribution system.
 8. A method for regulating a low voltageoutput from a power distribution system, comprising: providing the powerdistribution system, wherein the power distribution system comprises: aplurality of energy storage modules coupled in series; a plurality ofelectrically isolated power converters, each coupled across one or moreof the energy storage modules, the power converters when enabledconfigured to provide a low voltage output to an output of the powerdistribution system; and a control system controlling the powerconverters to provide the low voltage output; and selectively enablingone or more of the power converters to balance states of charge of theenergy storage modules, wherein balancing states of charge of the energystorage modules including by: monitoring a current output from a firstselectively enabled power converter, in the one or more selectivelyenabled power converters, in order to obtain a first power convertercurrent output; monitoring a current output from a second selectivelyenabled power converter, in the one or more selectively enabled powerconverters, in order to obtain a second power converter current output;enabling the first selectively enabled power converter for a firstduration of time; and enabling the second selectively enabled powerconverter for a second duration of time, wherein a first product of thefirst power converter current output and the first duration of timesubstantially equals a second product of the second power convertercurrent output and the second duration of time.
 9. The method of claim8, wherein selectively enabling power converters based on the currentoutput comprises: determining an amount of charge withdrawn from theenergy storage modules based on the current output from the powerconverters; and enabling one or more power converters corresponding toenergy storage modules having lower amounts of charge withdrawn.
 10. Themethod of claim 8, further comprising selectively enabling the powerconverters to maintain the current output within a desired operatingrange of the power converter.
 11. The method of claim 10, whereinmaintaining the current output within the desired operating range of thepower converter comprises: increasing a number of enabled powerconverters responsive to the current output being above an upperthreshold of the desired operating range; and decreasing the number ofenabled power converters responsive to the current output being below alower threshold of the desired operating range.
 12. The method of claim8, wherein balancing states of charge of the energy storage modules by:determining a state of charge of the energy storage modules; andselectively enabling one or more power converters corresponding toenergy storage modules having higher states of charge.
 13. The method ofclaim 8, wherein balancing power output of the energy storage modulescomprises: monitoring an amount of time each power converter is enabled;and selectively enabling the power converters for substantiallyequivalent amounts of time.